1. Field of the Invention
This invention relates to a process for producing photoconductive cadmium sulfide particles. More particularly, it relates to a process for producing cadmium sulfide particles having fine particle size and good sensitivity, with little agglomeration of the particles, by carrying out the first reaction of cadmium ion in an aqueous solution with hydrogen sulfide and then repeating the reaction operation of adding cadmium ion to the reaction mixture to cause the cadmium ion to react with hydrogen sulfide.
2. Description of the Prior Art
Photoconductive cadmium sulfide has so far been produced by passing hydrogen sulfide gas into a solution containing a water-soluble cadmium salt such as cadmium sulfate to form a cadmium sulfide precipitate. The cadmium sulfide produced in this way contains neither donor type nor acceptor type of impurity. In order to use this cadmium sulfide as raw material particles for an electrophotographic photoconductive material, it is necessary to fire it at a high temperature after addition of CuCl.sub.2 or the like as an activator and a halide compound such as CdCl.sub.2 or ZnCl.sub.2 as a flux. Cadmium sulfide is then activated by the diffusion of these impurities into the cadmium sulfide particles during the heat treatment. The cadmium sulfide thus activated is used as an electrophotographic photoconductive material.
In another conventional method of preparing raw particles of cadmium sulfide, as described in U.S. Pat. No. 4,021,237, hydrogen sulfide gas is caused to react with a solution containing cadmium ion and certain impurity element ion to produce cadmium sulfide particles containing an impurity. Since the raw particles thus prepared already contain a desirable impurity, the diffusion of the impurity into the particles occurs during the subsequent firing step even if a flux or the like is not added, and as a result, activated cadmium sulfide for electrophotography is obtained. In particular, in this method, the diffusion of impurity proceeds efficiently during the firing step because of previous incorporation of impurity in the cadmium sulfide, and excessive growth of cadmium sulfide particles does not take place since no flux is used.
However, the cadmium sulfide produced by these methods has still disadvantages in the following respects: Crystalline nuclei produced in the initial stage of the above reaction gather together as the reaction proceeds, and as a result, relatively large particles of higher order such as secondary particles are inadvantageously formed which contain two- or three-dimensional agglomerates of the primary particles.
When cadmium sulfide containing such agglomerates is dispersed in a binder resin and then coated on a substrate, minute uneveness is formed on the coating surface. A photosensitive member having such a coating surface is unsatisfactory when more improved images than those conventionally formed are aimed at. It is therefore necessary to minimize the two- or three-dimensional agglomeration of the particles as far as possible.
It has been so far expected that the reduced particle size of cadmium sulfide more improves the quality of its coating surface and permits more uniform arrangement of the particles on the surface of photosensitive member and further reduces the voids among the particles, thereby resulting in also a marked improvement in the resolving power of photosensitive member.
However, even when the particle size of cadmium sulfide is actually reduced by selecting appropriate reaction conditions, the two- or three-dimensional agglomerates of the particles are still produced. In the subsequent drying step, the cadmium sulfide particles become one solid mass as a whole which cannot be divided finely by a usual size reduction means except for mechanical means. When the reaction is carried out in a solution containing lower concentration of cadmium ion, cadmium sulfide particles are obtained which form two- or three-dimensional agglomerates thereof to a lesser extent. In this case, however, the production output decreases in consequence of low concentration of cadmium ion.